Commercial refrigerators lose efficiency when ice forms on the evaporator coils. To improve the efficiency the coils are defrosted on a periodic basis. This is accomplished by disabling the compressor and allowing or forcing the temperature of the coils to rise above the freezing temperature to remove the accumulated ice thereon.
This defrost function has been accomplished, in the past, by electromechanical timers that implemented time keeping with an AC synchronous motor clock that drove electrical contacts for the compressor and defrost means through mechanical linkages. The supply voltage requirements of the AC synchronous motor, however, dictated that separate designs or models of the timers be designed for 120 volt AC and 208˜240 AC supply voltage systems. Typically, these separate designs utilized different transformers with the appropriate turns ratio or a separate relay with the appropriate specified coil voltage for each input voltage range. As a result, the refrigeration service person was required to carry at least two different models for each service call, at an additional cost to him, in order to be prepared to replace which ever version of timer was installed at the customer location.
Another system that attempts to overcome this dual voltage problem is described in U.S. Pat. No. 6,563,237 to Bootz, for a “Multi-Voltage electromechanical time switch”, issued May 13, 2003. This patent describes an electromechanical timer designed to operate at either 120 Volt AC or 240 Volt AC by routing the supply voltage through a resistor divider. The customer is required to adjust the position of jumpers or switches, thereby changing the resistor divider ratio, to configure the timer for either 120 Volt AC or 240 Volt AC operation. Unfortunately, a mistake by the customer during configuration of the switches/jumpers could lead to damage or destruction of the timer.
Another solution for this problem is to use of a regulated DC voltage power supply to drive the relay coil. However, this is a costly approach which, in the highly cost competitive appliance industry, is unacceptable to the end consumer.
Other solution has been to sense the value of the bulk voltage and produce a high frequency pulse width modulated (PWM) signal that is used to drive the relay coil. The value of the duty cycle of the PWM signal is adjusted depending on the value of the bulk voltage so that the average value of the voltage across the coil is equal to its rated value. However, the power dissipated in the Zener diode placed across the relay is directly proportional to the switching frequency of the PWM. Therefore, high switching frequencies required for the coil to average the PWM signal translate to the requirement for high power, expensive Zener diodes. As such, this solution also undesirably increases the cost for the end consumer.
There exists, therefore a need in the art for a defrost timer mechanism that will operate at both 120 Volt and 208-240 Volt AC supply voltages without requiring user intervention.